JPS61284696A - Fuel support metal and nuclear reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a fuel support fitting and a nuclear reactor.

[Background of the invention]

A boiling water reactor generates steam in its core and supplies this steam to a turbine.

In boiling water reactors, improvements in nuclear thermal-hydraulic stability are being considered because steam voids rise within the reactor core. JP-A-57-52897 shows a configuration for improving nuclear thermal hydraulic stability in a boiling water nuclear reactor in FIG. 5. Its configuration will be explained below. In a boiling water type electronic reactor, a fuel support fitting is installed on the lower core support plate provided in the reactor pressure vessel, and the lower end of the fuel assembly is inserted into the upper opening of the cooling water passage provided in the fuel support fitting. The fuel assembly is supported by a fuel support fitting.

A round orifice having one opening is attached to the inlet of the cooling water passage in the fuel support fitting.

The amount of voids in the core in which the fuel assemblies are placed is measured, and the orifice shielding plate is operated to adjust the opening area of the orifice according to the amount of voids. When nuclear thermal-hydraulic instability occurs, the amount of voids in the fuel assembly increases, so when the amount of voids exceeds a predetermined amount, the orifice shielding plate is lowered to temporarily increase the opening area of the orifice. Increase flow rate and prevent increase in void volume. This eliminates nuclear thermal-hydraulic instability.

[Purpose of the invention]

An object of the present invention is to provide a fuel support fitting and a nuclear reactor that can prevent flow instability at low coolant flow rates and increase the reactor output control range based on the core flow rate without using any moving parts. be.

[Summary of the invention]

A feature of the present invention is that a resistance device having a coolant flow path in which a throat portion is formed is installed on a fuel support fitting, and the cross-sectional area of the coolant flow path is set upstream and downstream of the throat portion. In addition, the side wall of the coolant flow path is configured to have a smooth continuous surface without corners from the upstream side of the coolant flow path to the downstream side thereof. .

The present invention was developed based on the study of instability phenomena in conventional boiling water reactors. The details of the study will be explained below.

The instability phenomenon of the boiling water reactor occurs in the region above the curve U in FIG. That is, the instability phenomenon occurs in a region where the flow rate of cooling water passing through the reactor core (lower than c, referred to as core flow rate) is small and the reactor output is high. In FIG. 19, a curve (solid line) connecting points A and B shows changes in reactor output when the control rods are operated while the core flow rate is supplied through natural circulation. A straight line connecting point B and point 0 shows the change in reactor output when the degree of insertion of the control rods into the reactor core is held constant and the core flow rate is changed.

In this case, cooling water is forcibly supplied to the core by a recirculation pump. The dashed line characteristic in FIG. 19 shows the reactor power variation when the rated core flow rate of 20° is supplied by the recirculation pump.

The 0 point, which is the rated state (rion reactor output), is well out of the unstable region, so a stability margin is secured. However, the natural circulation state of curve AB (a state in which cooling water circulates due to the buoyancy of voids generated in the core) is close to the unstable region, so the stability margin is small.

In conventional boiling water reactors, in order to increase the stability margin under natural circulation conditions, a cooling water inlet is installed in the fuel support fitting 3 that supports the lower end of the fuel assembly 1, as shown in FIG. An orifice 4 is provided.

The effect of the orifice 4 under natural circulation conditions is shown in FIG. The horizontal axis in FIG. 21 shows the orifice coefficient indicating the flow resistance of the orifice 4, and the vertical axis shows the width reduction ratio which is an index of stability. As shown in Figure #c22, the attenuation ratio is the ratio of the amplitudes of adjacent sine waves (X2/XO), and represents the attenuation rate of fluctuations in flow rate, etc. The state shown in FIG. 22(A) that satisfies the equation (X2/xO)1) indicates an unstable state. Also, the 22nd
The state in Figure (B) shows a stable state. Considering the effect of the orifice 4 on this width reduction ratio, as shown in FIG. 21, the stability is improved as the flow resistance due to the C orifice 4 increases as the orifice coefficient increases. Stability becomes a problem in the natural circulation state where the reduction ratio approaches 1.0, as shown by the solid line in Figure 23 (Bl), especially in the vicinity of point B in Figure 19. The orifice 4 of the metal fitting 3 has a constant orifice coefficient as shown by the solid line in FIG. 23(A) even if the reactor power changes as the core flow rate increases as shown by the solid line in FIG. 23(C).

In the orifice 4 having such an orifice coefficient,
The width reduction ratio changes as shown by the solid line in FIG. 23(B) described above. As shown in FIG. 23, as the orifice 4 of the conventional fuel support fitting 3, an orifice having an orifice coefficient determined on the premise of avoiding an unstable state in a natural circulation state is used. This orifice coefficient is the 23rd
As shown in the figure (Al), it is always constant.

The inventors succeeded in reducing the core flow rate by providing the fuel support with a mechanism that increases the pressure loss during the natural circulation of cooling water when the core flow rate is low and decreases the pressure loss when the core flow rate is large, without using any moving parts. Focusing on the ability to prevent unstable conditions when the flow rate is low (during natural circulation) and increase the control range of reactor output by controlling the core flow rate, various studies and experiments were conducted to find the specific structure of the above-mentioned mechanism. Ta. As a result of the study and experiments, the orifice 5 having the opening shape shown in FIGS. 24A and 24B was created.
It was found that the above conditions were satisfied.

The orifice 5 is constructed by arranging three round rods 7 in parallel in a ring 6 with a gap 8 between them, and connecting both ends of the round rods 70 to the ring 6.
The coolant flows through the gap @5 in the direction of the arrow PL. The cross-sectional shape of the gap 8, that is, the cross-sectional shape of the coolant flow path in the direction of the arrow PL, is such that the cross-sectional area gradually decreases in the flow direction upstream of the narrowest throat part 8A, and decreases in the flow direction downstream of the throat part 8A. The cross-sectional area gradually increases, and the wall surfaces (surfaces of the round bar 7) on both sides forming the gap 8 become continuous surfaces without corners. The shape of the gap 8 in the axial direction of the round bar 7, particularly the shape of the throat portion 8A, is elongated, determined by the axial shape of the side surfaces of the pair of opposing round bars 7. The flow characteristics of the orifice 5 were obtained using the experimental apparatus 9 shown in FIG. The schematic structure of the experimental apparatus 9 will be explained. The experimental apparatus 9 has a tank 10 filled with water, a pump 11, a flow 11 regulating valve 13, and a test specimen 14 connected through piping 15. One end of the pipe 15 is open above the tank 10. The pump 11 is driven by a motor 12. The orifice 5 shown in FIG. 24A is installed within the test specimen 14. The flow rate and temperature of water supplied into the test body 14 are measured by a flow meter 16 and a thermometer 17. The differential pressure between the upstream side and the downstream side of the test specimen 14 is measured by a differential pressure gauge 18.

In the flow test of the orifice 5, the pump 11 is driven to supply water in the tank 10 through the pipe 15 to the orifice 5 in the test body 14. The flow rate of water to be supplied is determined by the flow rate control valve 1.
Controlled by 3.

When the measurement data of flow rate, differential pressure, and temperature obtained in this experiment were organized, the characteristics shown in FIG. 26 were obtained. The horizontal axis in FIG. 26 shows the Raynozzle number Rex, and the vertical axis shows the orifice coefficient Kat. The orifice coefficient Key of the orifice 5 is approximately 77 near Re=13 1 (19th
Re=45×10', which corresponds to point 0 in the figure, is about 60. The Oriais coefficient KmW at the 0 point in Figure 19 is 4
It is about 22 inches lower than that at point B in Figure 19.

Therefore, the main power loss at point 0 is about 2 more than that at point B.
2 points lower. The orifice coefficient of orifice 5 is approximately 7 with Rle in the range of 13X10' to 30X10'.
It is almost constant at 7! >, ee gradually decreases when it exceeds 30×104. The point Re=30X10' is the first
In FIG. 9, this corresponds to a time when the core flow rate is approximately 601.

The orifice 5 to which a plurality of round bars are attached in this way has no moving parts, but may have the function of decreasing the orifice coefficient as the flow rate increases (that is, reducing the pressure loss as the flow rate increases). are doing.

Figures 27 and 28 explain why the characteristics shown in Figure 26 occur.
This will be explained based on the diagram. FIG. 27 shows the flow state of water between the adjacent round rods 7 of the orifice 5 in the natural circulation state of FIG. 26 at Re=13X10'.

Water is flowing in the direction of arrow FL. In this case, round bar 7
The boundary layer formed along is a laminar boundary layer. The separation point is a position where the boundary layer separates from the surface of the round bar 7, and is formed on the upstream side of the flow from a perpendicular line passing through the axis of the round bar 7. Separation zone moths are formed downstream of the separation point. The pressure in the mesh region formed on the downstream side of the round bar 7 is lower than the pressure in the area where there is no separation of the boundary layer on the upstream side of the round bar. Therefore, a pressure difference occurs between the upstream side and the downstream side of the gap 8 formed between the adjacent round bars 7, and a pressure loss occurs between the upstream side of the gap 8 and the downstream side thereof.

FIG. 28 shows the flow state of water between the adjacent round bars 70 of the orifice 5 at Re=45×104 at the 0 point in FIG. In this case, the boundary layer formed along the round bar 7 is a turbulent boundary layer. The peeling point is
It is formed on the downstream side of the flow from a perpendicular line passing through the axis of the round rod 7. The delamination region formed downstream of the delamination point is the second
This decreases compared to the case shown in Figure 7. Therefore, the low pressure region (separation region) formed behind the round bar 7 in the flow direction is reduced compared to the case of FIG. 27, and the pressure difference between the upstream and downstream sides of the gap 8 is also reduced. . The pressure loss between the upstream side and the downstream side of the gap 8 in FIG. 28 is also reduced compared to that in FIG. 27.

The flow within the boundary layer formed along the round bar 7 changes from laminar flow to turbulent flow as the flow velocity in the gap 8 increases, that is, the Reynolds number Re in the gap 8 increases. The boundary layer formed along the round bar 7 changes from a laminar boundary layer to a turbulent boundary layer at the point in FIG. 26 where the Oriais coefficient Kit starts to transition from 77, that is, the point Re'v33X10'. occurs in This Reynolds number of 33×10′ is called the critical Reynolds number ec. Re (circle II in the area of Rec
The boundary layer at point 7 is a laminar boundary layer, and the pressure loss between the upstream and downstream sides of gap 8 is large. The pressure loss between the upstream and downstream sides of I8 is reduced.

The orifice coefficient of orifice 25 tends to be 23rd.
As shown by the broken line in the figure (person), the Oriice coefficient decreases as the core flow rate increases (assuming that the orifice coefficient decreases from the natural circulation state at point B). Due to such a decrease in the orifice coefficient, the width reduction ratio of the orifice 25 increases by nine compared to the orifice 4, as shown in FIG. 23(B). However, the rate of increase is small, and the orifice 250 width reduction ratio never exceeds 1.0.

Using rods 7A to 7C having cross-sectional shapes as shown in FIG. 29 in place of the round rod 7 of the orifice 5,
A flow experiment similar to that for orifice 5 was also conducted for each orifice assembled and attached to ring 6 as shown in (F). In these orifices as well, the boundary layer formed along rods 7 to 7C is similar to orifice 5.
e (Re c is not a laminar boundary layer, 几e) Rec is a turbulent boundary layer. The critical Reynolds number ec is the second
9 (A) to (F) are different from each other. Water was allowed to flow in the direction of arrow PL for each rod combination shown in FIG. In FIG. 29(A), seven rods each having an elliptical cross section are arranged so that the long axis of the ellipse is perpendicular to the force direction of the arrow PL. In FIG. 29(B), the rod 7A is arranged so that the long axis of the ellipse is in the direction of the force indicated by the arrow FL.

Fig. 29 (C1) shows a rod 7B with a square cross section arranged so that one side of the square is in the direction of the force indicated by the arrow FL. It is arranged so as to be inclined at 45 degrees with respect to the L direction.Second
Figure 9 (F,) shows a bar 7C with an equilateral triangular cross section.
It is arranged so that the two corners face upstream. Second
In FIG. 9(F), 117c is arranged so that one corner of the cross section faces downstream. The corners of the cross sections of the rods 7B and 7C are not corners, but have smooth curved surfaces with arbitrary curvatures.

The rods shown in FIG. 24A and FIGS. 29(A) to (F), that is, the side walls of the coolant flow path formed between the resistors (side surfaces of the resistors) are shown in FIGS. 27 and 28. It is formed of a continuous smooth surface without corners in the fluid flow direction so that the separation point moves from the upstream side to the downstream side of the throat portion 8A. If a corner is formed on the side wall of the coolant flow path in the fluid flow direction, a separation point will occur at the corner, and even if the flow velocity increases, the separation point will move downstream from the corner. It disappears. For this reason, the characteristic shown in FIG. 26, that is, the phenomenon in which the pressure loss decreases in the region of Re>Rec cannot be obtained.

By changing the cross-sectional shape and dimensions of the resistor C (rods shown in Figures 24A and 29 (A) to (F')), the flow rate of the coolant changes depending on the area of the flow path constricted by the resistor. Pressure loss can be adjusted. The width Wl of the 70-tooth portion 8A formed between the resistors is determined in the flow direction P of the coolant.
It is narrower than the width Wt of the resistor in the direction perpendicular to L.

Based on these study results, the inventors have determined that the coolant flow path (having a throat portion) of the resistance device (for example, the above-mentioned orifice 5) provided on the fuel support fitting has a cross-sectional area of the coolant flow path. The coolant is constructed so that it increases continuously from the throat to the upstream and downstream sides, and the side wall of the coolant flow path is configured so that the separation point can be moved between the upstream and downstream sides of the throat. , they came to the conclusion that it should be composed of smooth, continuous surfaces with no corners. An embodiment of the present invention that satisfies these conditions will be described below.

[Embodiments of the invention]

A preferred embodiment of the present invention applied to a boiling water reactor will be described below.

4-4 show examples of fuel support fittings. The fuel support fitting 20 includes a fitting body 21 having four cooling water passages 22 therein, and an inlet opening 2 of the cooling water passages 22.
3 and an orifice 25 attached thereto. Orifice 25 is a resistance device. As shown in FIGS. 3 and 4, the orifice 25 has thirteen round rods (or circular tubes may be used) 27 each having a circular cross section attached to a ring 26 in parallel at intervals. A gap 28 is formed between adjacent round bars 27 to serve as a cooling water flow path.

8A is the throat portion where the width of the gap 28 is the narrowest. The gap 28 in this embodiment has a cross-sectional area that gradually increases toward the upstream and downstream sides of the throat section, and has a pair of side walls (adjacent and (the side surface of the round bar 27). Orifice 25
has a plurality of such gaps 28.

The metal fitting main body 21 has a ten-rod-shaped through hole 29 extending in the axial direction at the center thereof. Although not shown, this through hole 29 is a hole into which a control rod is inserted. Each cooling water passage 22 is arranged around the through hole 29.

FIG. 5 shows an actual example of a boiling water reactor to which the fuel support fitting 20 is applied.

The boiling water reactor 30 includes a fuel support fitting 20. It has a reactor vessel 31, a core lower support plate 33, and a fuel assembly 37. A core shroud 32 is disposed within and attached to the reactor vessel 31 . A jet pump 35 is installed between the reactor vessel 31 and the core shroud 32. The core lower support plate 33 is attached to the lower end of the core shroud 32 and
Located within 2. A plurality of fuel support fittings 20 are installed on the core lower support plate 33 by penetrating the core lower support plate 33. The orifice 25 is located below the core lower support plate 33 and opens into the lower plenum 36. A large number of fuel assemblies 37 arranged within the core shroud 32 are arranged in a first
As shown, the lower tie plate 38 is supported by being inserted into the outlet opening 24 of the cooling water passage 22 of the fuel support fitting. The upper end of the fuel assembly 37 is installed within the core shroud 32 and supported by the upper grate plate 34 . A portion of the core shroud 32 in which the fuel assemblies 37 are arranged is the core.

A lower plenum 36 is formed within the reactor vessel 31 below the core lower support plate 33 . A number of control rod guide tubes 39 are installed within this lower plenum 36 . The control rod drive housing 40 is joined to the lower end of the control rod guide tube 39 and extends downward through the reactor vessel 31 . The control rods 41 move vertically within the control rod guide tubes 41 and are inserted into and removed from between the fuel assemblies 37 in the reactor core via the through holes 29 of the fuel support fittings 20. The control rod 41 is operated by a control rod drive device C (not shown) installed in the control rod drive device housing 40.

The boiling water reactor 30 is started up as follows. The boiling water reactor 30, which has reached a critical state after the control rod 41 is withdrawn from the reactor core, performs temperature and pressure increasing operation without further withdrawing the control rod 41. At this time, cooling water is supplied to the core. This cooling water supply is provided by a recirculation pump (not shown)
This is done by driving the jet pump 35 to eject cooling water from a nozzle into the jet pump 35. Cooling water is squirted out,
Cooling water between the reactor vessel 31 and the core shroud 32 is sucked into the jet pump 35. The cooling water discharged from the jet pump 35 flows into the lower blennium 36,
The water flows into the cooling water passage 22 of the fuel support fitting 20 through the orifice 25 and is supplied to the fuel assembly 37 through the outlet opening 24 . When the cooling water passes through the orifice 25,
Pass through Sekiwt28.

The pressure and temperature inside the reactor vessel 31 are at predetermined values (approximately 70 kg
/i and approximately 280CI, the flow rate of cooling water discharged from the recirculation pump is maintained at the flow rate of the operating state of 20cm/p, and the control 11m41 is withdrawn from the core. At this time,
The reactor power increases as shown by the broken line in FIG. When the reactor output reaches point D, the withdrawal operation of control #141 is stopped. If the control rod 41 is withdrawn in a region where the reactor output exceeds point D, there is a risk that the fuel element in the fuel assembly 37 will be damaged due to mutual mechanical interference between the pellets and the cladding tube. There is sex. Reactor output up to D point 9
In order to raise it to a high state, the control rods 41 are stopped and held in the ratio state without moving, and instead the cooling water gf (core flow rate) supplied to the reactor core is increased. Increasing the core flow rate can be achieved by increasing the recirculation pump rotation speed. As the core flow rate increases, the reactor power increases from point D to point C along the line aBC shown in FIG. When the reactor power reaches the zero point, the increase in core flow rate is stopped. The reactor power decreases by 1001 as fuel is consumed. To compensate for this and maintain reactor power at 100%, core flow is increased. Compensation of reactor power due to increase in core flow rate is
There is a limit due to the capacity of the recirculation pump. When this limit is reached, the reactor core flow rate is lowered to lower the reactor output to below point D, and the reactor output is increased to point D by the withdrawal operation of control 1m41. Thereafter, the reactor output is increased to CI# along the straight line BC as described above.

Similarly to the orifice 5, the orifice coefficient of the orifice 25 decreases as the Reynolds number of the gap 28 increases, as shown in FIG. The critical Reynolds number Rec (transition start point of the orifice coefficient Kit) of the orifice 25 is 40×10'. The orifice 250 gap 28 is
In the region of Re(40x10', a laminar boundary layer as shown in Fig. 27 is formed, and in the region of Re > 40x104, the laminar boundary layer
A turbulent boundary layer as shown in the figure is formed. The diameter of the round bar 27 of the orifice 25 is 0.735c*, and the total projected area Aob of the round bar 27 attached to the ring 26 is 89.
．． 3CI! The total projected area Ar of the gap -28 formed between the round bars 27 is 13.2 m, and the area inside the ring 26 is 10Z5cn (Aob+Ay).

In the boiling water reactor 30, when the core flow rate increases and the Reynolds number Re exceeds the critical Reynolds number Rec, the pressure loss in the orifice 25 begins to decrease. The pressure loss when the reactor power is 10 (l) (point 0 in Figure 19) is D
This is approximately 22 inches less than the pressure drop at point. Therefore, in this embodiment, the flow rate of the cooling water discharged from the recirculation pump at point C in FIG. At point C in FIG. 19 in the furnace, the flow rate of cooling water is reduced compared to the flow rate of cooling water discharged from the recirculation pump. The capacity of the recirculation pump in this example is equal to its capacity in a conventional boiling water reactor.

Therefore, in this embodiment, the amount of increase in the core flow rate after reaching the zero point state in FIG. 19 is greater than in the conventional case. That is, in this embodiment, the core flow rate can be increased as indicated by the broken line LI in FIG. 23(C) (compared to the conventional art). Therefore, in this embodiment, the ratio at which the reduction in reactor power due to fuel consumption can be compensated for by increasing the reactor core flow rate, that is, the control range of the reactor power by the core flow rate, can be increased compared to the conventional method. This lowers the reactor power below point D and controls the control rod 41.
This leads to a significant reduction in the number of times the extraction operation is performed, and the capacity utilization rate of the nuclear reactor in this embodiment can be significantly increased compared to the conventional one.

If the recirculation pump trips while the reactor is operating at 100% power, the core flow rate will decrease and the cooling water in the core will naturally circulate. As the core flow rate decreases, the reactor output decreases along the straight line BC in FIG. 19, and decreases to the output at point B. In this way, when the core flow rate reaches the flow rate of the natural circulation state, between the orifice 25 @28
The flow state in the orifice 25 is as shown in FIG. 27, and the orifice coefficient Kor of the orifice 25 increases. Therefore, in this embodiment, the unstable phenomenon can be prevented while the core flow rate is in a natural circulation state.

In this way, by using the orifice 25 having no movable parts, this embodiment can prevent instability at low flow rates and improve the capacity utilization rate of the reactor.

In this embodiment, since a plurality of round rods 27 are arranged in parallel, the boundary layer separation conditions are determined by the flow rate of the cooling water flowing through the gap 28, and the conditions on the upstream side of the orifice 25, that is, in the lower plenum 36. not influenced by (9) In this embodiment, a plurality of round rods 27 are arranged in the opening portion 23 of the cooling water passage 220 of the fuel support fitting 20 to form a plurality of gaps 28. The flow direction of the cooling water flowing into the cooling water passage 22 is forcibly corrected, and the generation of vortices within the cooling water passage 22 is reduced. This is also a factor in preventing flow instability. In particular, since the cooling water in the lower plenum 36 passes between a large number of control rod guide tubes 39 arranged in the lower plenum 36 and reaches the fuel support fitting 20, the cooling water flows through the control rod guide tubes 39 between them. The flow is significantly disturbed. Therefore, cooling water flows into the orifice 25 provided in the fuel support fitting 20 from all directions. Since the orifice 25 has a plurality of gaps 28 formed by the plurality of round bars 27, it has an extremely large effect of correcting the flow of cooling water flowing in from all directions into a flow in a specific direction.

As for the shape of the orifice installed in the fuel support fitting 20, various shapes other than the orifice 25 can be considered. Other embodiments of the orifice will be described below.

In the orifice 25A shown in FIGS. 6A and 6B, a ring 27A having a circular cross section is arranged concentrically with a resistor in place of the round bar 27 of the orifice 25. ring 2
7A is fixed to the ring 26 by a support member 29. Orifice 25A also has the same function as orifice 25.

The orifice 25B shown in FIGS. 7A and 7B uses rods 27B with an elliptical cross section instead of the resistor, and three rods 27B are arranged in parallel. The barrel 27B has an ellipse whose long axis direction is perpendicular to the direction of arrow FL (flow direction). FIG. 29 (This is the arrangement of Al. Also, the ratio of the major axis to the minor axis of the ellipse is 2, and the diameter is λ8cIn. This orifice 25B can perform the same function as the orifice 25, and can also transition at a high flow rate. If you want to wake up (Rec
= 1.6X10'). Furthermore, since the number of rods 27B is small, the orifice 25B can be manufactured easily.

It is also possible to use the rod 2733 by arranging it so that the major axis direction of the ellipse is parallel to the flow.

A characteristic of this case is that the pressure loss in the orifice gradually decreases near the critical Reynolds number. Furthermore, the pressure loss of the orifice formed by attaching the resistor 27C shown in FIG. 8 to the ring 26 is also gradually reduced. The F section of the resistor 27C is arranged toward the upstream side of the flow.

The orifice shown in FIGS. 9A and 9B uses a rod 27D having a triangular cross section with smooth corners as a resistor, and this rod 27D is attached to a ring 26.

The orifice 25D is provided with a hole 2 as shown in FIG. 29(F).
Set the direction so that one of the vertices of the 7D cross section is upstream. The length and height of the base of rod 27D are also 5c1F!
It is.

The radius of each corner of the cross section of 1127D is 4 times the length of the base.
It is rounded with a curvature of about 1/2.

The orifice 25D has eight 1I27D arranged in parallel. The critical Reynolds number of orifice 25D is approximately 6X1
It will be 05. This embodiment has the advantage that the transition conditions can be varied by changing the curvature of the corner portion. Orifice 25D has the same function as orifice 25.

The orifice 25g shown in FIGS. 10A and 10B is
A resistor 27E formed by arranging a plurality of round bars in a grid pattern is used, and the resistor 27E is attached to a ring 26. A perspective view of the resistor 27g is shown in FIG. 10C. Orifice 2
5E has the advantage of being superior in strength. Orifice 25E also has the same function as orifice 25. Furthermore, orifice 25E has the same function as orifice 25. Since the orifice 25g has a longer wetted length than the first orifice 25, it is easy to increase the Reynolds number. Therefore, a transition in the orifice coefficient Kar occurs at a very low flow rate.

It is also possible to make the ring 26 of the orifice 25B rectangular.

The orifice 25F shown in Figure 11 and Figure 11B is
It has the same components as the orifice 25. but,
In the orifice 25, the space 1!128 between the round bars 27 is not uniform, being wide at the center and narrowing on both sides, as shown in FIG. 11B. By making the gaps 28 between the round bars 27 non-uniform, the Reynolds number is distributed in the direction perpendicular to the flow direction of the orifice 25F, and the pressure loss is gradually reduced. A nuclear reactor using orifice 25F is
Very stable operation is possible. Orifice 25F' also
It has the same function as orifice 25.

The orifice 25G shown in Figure 12 and Figure 12B is
It is a good idea to attach to the ring 26 a resistor 27G in which the gaps 928 between the resistors 27E of the orifice 25H are made uneven in the same manner as the orifice 25F.

The orifice 25H shown in FIGS. 13A and 13B is
It has the same components as orifice 25. In the orifice 25H, the interval between the round rods 27 is narrowed on one side and widened on the other to create a flow velocity distribution.

The orifice 25I shown in Figure 14 and Figure 14B is
The inside of the ring 26 has a smoothly curved cross-sectional shape, and has a structure with one opening 28 in the center. The orifice 25I has advantages such as a simple structure, excellent strength, and easy manufacture. The absolute magnitude of the resistance of the orifice 25I can be changed by changing h.
Furthermore, by changing the radius of curvature r, it is possible to change the Reynolds number. However, orifice 25I is
Compared to the orifice 25, the function of forcibly correcting the flow direction of the cooling water flowing into the opening 28 is weaker, and the generation of vortices in the cooling water passage 22 of the fuel support fitting 20 is lower than that of the nine orifices described above. There will be more. Therefore, the stability of the flow deteriorates somewhat.

Orifices 25 and 25-25I are cooling water passages 22
The fuel support fitting 20 can be attached anywhere within the fuel support fitting 20. In that case, the function of each orifice remains unchanged. The nine resistors provided in each orifice may be directly attached to the metal fitting body 21 of the fuel support fitting 2o within the cooling water passage 22 without using the ring 26.

Orifices 25, 25E and 25G are complex in structure. Also, the orifices 25B and 25D are difficult to manufacture because of their resistor rods. Round rods are the easiest to manufacture as resistors, and they are also readily available. This ninth 1. It is easiest to construct an orifice using a round rod (or circular tube) as a resistor.

In the embodiment described above, the orifice 25 is connected to the fuel support fitting 2.
Although the orifice 25 is attached to the fuel assembly, the same effect as the embodiment shown in FIG. 1 can be obtained even if the orifice 25 is attached to the fuel assembly. This embodiment will be explained based on FIGS. 15 and 16.

The fuel assembly 45 of this embodiment has a channel box 56
, a lower tie plate 38, an upper tie plate 46, a spacer 54, a fuel rod 47, and an orifice 25. The upper and lower ends of the fuel rods 47 are held by the lower tie plate 38 and the upper tie plate 46. The water rod 53 is also held at both ends by the lower tie plate 38 and the upper tie plate 46. The spacer 54 is connected to the fuel rod 4
Several fuel rods 47 are arranged in the axial direction to maintain an appropriate gap between the fuel rods 47. channel box 56
is attached to the upper tie plate 46 and spacer 54
It surrounds the outer periphery of the bundle of fuel rods 47 held by the fuel rods 47 . An orifice 25 shown in FIGS. 3 and 4 is provided in the lower tie plate 38, and a space 57 serves as a passage for cooling water.
is located at the lower end of the unit and attached to the lower tie plate.

FIG. 16 shows the detailed structure of the fuel rod 47. fuel rod 47
In this case, a large number of fuel pellets 51 are loaded into a cladding tube 48 with both ends sealed by a lower end plug 49 and an upper end plug 50. A spring 52 is disposed within the gas blemish within the cladding tube 48 and urges the fuel pellet 51.

FIG. 17 shows an example of a boiling water reactor in which a fuel assembly 45 is arranged in the core. FIG. 17 shows a structure corresponding to the engineering section in FIG. 5. The boiling water reactor of this embodiment has the same structure as the boiling water reactor 30 shown in FIG. 5, except for the structure of this engineering section. The fuel support fitting 3 is installed on the core lower support plate 33 similarly to the fuel support fitting 20. The fuel support fitting 3 has a structure shown in FIG.
It has a through hole 29 in the center thereof and four cooling water passages 22 around the through hole 29. The inlet opening 23 of the cooling water passage 22 is located below the core lower support plate 33 and is open to the lower blemish 36 .

The fuel assembly 45 is held by the fuel support fitting 3 by inserting the lower end of the lower tie plate 38 into the outlet opening 24 of the cooling water passage 22 .

The cooling water in the lower brenum 36 flows into the cooling water passage 22 of the fuel support fitting 3 through the inlet opening 23 and is supplied into the fuel assembly 45 through the orifice 25 . The orifice 25 functions in the same manner as in the previous embodiment when attached to the fuel support fitting 20. That is, orifice 25
In this case, a laminar boundary layer and a turbulent boundary layer as shown in FIGS. 27 and 28 are formed according to changes in the core flow rate, and the pressure loss changes.

In this embodiment as well, the same effects as in the ninth embodiment described above can be obtained. The above-mentioned orifice 2 is substituted for orifice 25.
5A to 25I may be attached to the fuel assembly 45 at the same position as the orifice 25.

When orifice 25It- is used, the rectifying effect of the cooling water is smaller than that of other orifices.

〔Effect of the invention〕

According to the present invention, instability of a nuclear reactor in a low flow rate state of coolant can be prevented by using a resistor without a moving part. Furthermore, the range of control of the reactor output based on the core flow rate can be expanded, and the capacity utilization rate of the reactor can be improved.

[Brief explanation of the drawing]

FIG. 1 is an enlarged view of the construction part shown in FIG. 5, and is a vertical cross-sectional view of a fuel support fitting according to a preferred embodiment of the present invention, and FIG. 2 is a perspective view of the fuel support fitting shown in FIG. 1. , Fig. 3 is a front view of the orifice shown in Fig. 1, Fig. 4 is a sectional view taken along the line ■-■ of Fig. 3, and Fig. 5 is an implementation of a boiling water reactor to which the fuel support fittings shown in Fig. 1 are applied. Local longitudinal sectional views of examples, FIGS. 6A, 7A, and 9th human figure. Figure 10, Figure 11A, Figure 12, Figure 13A and Figure 14 are front views of other embodiments of the orifice, Figure 6B.
The figure is an Xr Xt sectional view of the 6th figure, Figure 7B is a x3-Xs sectional view of the figure,
Figure 10B is the 10th person figure 0X4X4 flH1ji figure,
il OC diagram H gl A local perspective view of the resistor 27g in the OA diagram, Figure 11B is the x, -Xs sectional view of Figure 11A,
Figure 2B is Figure 12A (DXs Xam view, Figure 13B
The figure is an Xr-Xr sectional view of the 13th human figure, FIG. 14B is an Xs-X@ sectional view of FIG. is the first
Figure 5 is a structural diagram of the fuel rod, Figure 17 is a structural diagram showing the fuel assembly shown in Figure 15 installed in the core of a boiling water reactor, and Figure 18 is a structural diagram of the fuel support fittings shown in Figure 17. A perspective view, FIG. 19 is a characteristic diagram showing the relationship between core flow rate and reactor output,
Fig. 20 is a partial vertical sectional view of a conventional fuel support fitting;
The figure is a characteristic diagram showing the relationship between the orifice coefficient and the width reduction ratio, Figure 22 is an explanatory diagram showing the concept of the width reduction ratio, and Figure 23 shows the changes in the Orifice coefficient, width reduction ratio, and reactor power with respect to the core flow rate. The characteristic diagram shown in Fig. 24A is a front view of the orifice test specimen used in the present invention, Fig. 2+B tri Fig. 24A 0V
-V sectional view, Figure 25 is a structural diagram of the experimental equipment for conducting the flow experiment of the orifice shown in Figure 24, and Figure 26 is the characteristic showing changes in the orifice coefficient based on the flow experiment results for the orifice shown in Figure 24. Figure 27 is an explanatory diagram showing the flow state in the region of R e < Rec in the orifice gap of Figure 24A, and Figure 28 is an explanatory diagram showing the flow state in the region of R, e > Rec in the orifice gap of Figure 24. FIG. 29 is a cross-sectional view of another resistor for which flow experiments were conducted. 3.20...Fuel support fitting, 21...Cooling water passage,
25... Orifice, 27... Maruzen, 28... Gap, 30... Boiling water micronuclear reactor, 31... Reactor vessel, 32... Core shroud, 33... Core lower part Support plate, 36... Lower plenum, 37.45... Fuel assembly, 38... Lower tie plate, 46... Upper tie plate.

Claims (1)

[Scope of Claims] 1. Consists of a main body having a coolant passage therein, and a resistance device having a coolant passage provided in the coolant passage and having a throat portion formed therein; The cross-sectional area of the coolant flow passage increases continuously from the throat part toward the upstream and downstream sides, and the side wall of the coolant flow passage has an angular cross-sectional area from the upstream side of the throat part to the downstream side thereof. A fuel support fitting characterized in that it is composed of a continuous surface with no parts. 2. The resistance device has a plurality of resistance members, a plurality of the coolant channels are formed between adjacent resistance members, and a side wall of the coolant channel is formed between the resistance members. The fuel support fitting according to claim 1, which is a surface. 3. A reactor vessel, a core support member, having a coolant passage therein and a resistance device provided in the coolant passage, and a fuel support fitting installed in the core support member, and the fuel a fuel assembly whose lower end is inserted and held in the coolant passage of a support fitting, the resistance device has a coolant flow passage in which a throat portion is formed, and the resistance device has a coolant flow passage in which a throat portion is formed; The cross-sectional area of the coolant channel increases continuously from the throat section toward the upstream and downstream sides, and the side wall of the coolant channel extends from the upstream side of the throat section to the downstream side thereof. A nuclear reactor characterized by being composed of no continuous surface. 4. The resistance device has a plurality of resistance members, a plurality of the coolant channels are formed between adjacent resistance members, and a side wall of the coolant channel is formed between the resistance members. The nuclear reactor according to claim 3, which is a surface.